Thailand's Deputy Prime Minister and Defence Minister Prawit Wongsuwan attends during an announcement the junta's two year accomplishments at Government House in Bangkok, Thailand, September 15, 2016.Athit Perawongmetha/File Photo

BANGKOK (Reuters) - Thailand's junta came under scrutiny this week after critics filed a petition asking the office of the auditor-general to investigate allegations of extravagant spending on a trip to Hawaii for a defence meeting.

It is the latest in a series of allegations against the military government that seized power in May 2014, promising to root out entrenched corruption in state institutions and close Thailand's festering political divide.

The government has defended allegations that a 20.9 million baht ($600,000) chartered flight taken by Defence Minister Prawit Wongsuwan and his entourage to a meeting in Hawaii last week was exorbitant.

On Wednesday, Srisuwan Janya, head of the Association for the Protection of the Constitution, a government watchdog, petitioned the Office of the Auditor-General to investigate expenses incurred on the Thai Airways flight.

These expenses included 600,000 baht ($17,200) spent on in-flight food and beverages, according to details posted on the Secretariat of the Prime Minister's website.

The Office of the Auditor-General on Friday said it had scrutinised expenses incurred on the Hawaii trip and found nothing wrong.

"We found no wrong doing in Defence Minister Prawit Wongsuwan's trip to America," Auditor-General Pisit Leelavachiropas told reporters, adding that his office was still waiting for details of the total flight cost from Thai Airways.

The allegations threaten to erode the military government's credibility, say critics, including civil society groups.

Last month, Isra News, an investigative news website, reported that Pathompol Chan-ocha, a nephew of junta chief Prayuth Chan-ocha, was awarded seven construction projects with the Third Army Region, which had been under his father's command, prompting claims of nepotism.

The National Anti-Corruption Commission is currently investigating those claims.

Preecha Chan-ocha, Pathompol's father and junta chief Prayuth's brother, has defended his son and said he acted according to the army's rules and regulations for contractors.

"Both of these are issues that Thai society is criticising a lot and it will reduce the junta's credibility," Srisuwan told Reuters. "This is contrary to what people expect from this military government. The prime minister should boldly act to restore public confidence and not shirk responsibility."

Colonel Piyapong Klinpan, deputy spokesman for the junta, formally known as the National Council for Peace and Order (NCPO), said it welcomed public scrutiny.

"If there is any issue of public interest, then the agency involved can examine it. We are not trying to hide or conceal anything," Piyapong told Reuters.

"The NCPO does not interfere in these independent bodies and we ask that people trust the NCPO."

The investigation follows allegations of graft last year, levelled by some Thai media and opposition groups, involving construction of a $28 million park built to honour the monarchy that threatened to undermine an anti-graft drive by the junta.

An internal investigation by the army in November 2015 found no evidence of corruption.

Thailand's military has always been powerful, but the 2014 coup established it as the nation's pre-eminent institution.

Thais voted overwhelmingly in August to accept a junta-backed constitution that the government says is designed to heal more than a decade of divisive politics. Critics of the government, including major political parties, say the charter will enshrine the military's role for years to come.